In the production of paper pulp, there are a number of circumstances in which the comminuted cellulosic fibrous material to be bleached to produce the pulp has cleanliness which varies significantly over time. For example, one of the primary concerns when attempting to recover usable material from recycled paper is the uncontrollable variability of the quality and cleanliness of the furnish (waste paper) supplied. Paper collected by waste collectors and delivered to recycling mills is rarely screened to provide a uniform quality of furnish, and the cost of screening the paper manually or by automated sorter can significantly increase the price of the waste paper. The problem with the variability of the furnish is that it directly affects the quality of the pulp produced, and the cost of bleaching chemical is directly related to the cleanliness of the furnish. Depending upon the quality (typically its brightness or kappa number) of the furnish different bleaching chemical charges are necessary.
In conventional pulp mills, changes in brightness and kappa are limited, and variations of chemical charges can be made relatively easily utilizing conventional closed loop controls. However the unpredictability of recycled furnish is not amenable to simple closed loop control of chemical charges, and may require significantly different bleaching sequences or vastly different chemical charges or treatments.
According to the present invention, a method and apparatus are provided which provide the versatility necessary to a bleaching operation to accommodate variations in cleanliness of the furnish over time. The method of bleaching according to this aspect of the invention utilizes at least first and second different bleaching stages, using first and second different effective bleaching chemicals. While non-chlorine chemicals (e.g. oxygen, peroxide, ozone, formamidine sulfinic acid (FAS), hydrosulfite, peracetic acid, and enzyme treatments are preferred, the invention is not limited to non-chlorine bleaching chemicals but is also applicable to chlorine, chlorine dioxide, or equivalent bleaching chemicals. This aspect of the method according to the present invention comprises the following steps: (a) Sensing the brightness, color or lignin content (or any combination thereof) of the comminuted cellulosic fibrous material prior to treatment in the first bleaching stage. (b) Controlling the quantity of first bleaching chemical added to the first stage in response to the sensing from step (a) to achieve a target brightness increase for treatment of the material in the first stage. (c) Sensing at least one of the brightness, color or lignin content of the comminuted cellulosic fibrous material after treatment in the first bleaching stage and before treatment in the second stage. And, (d) controlling the quantity of second bleaching chemical added to the second stage in response to the sensing from step (c) to achieve a target brightness increase for treatment of the material in the second stage. The invention also relates to the cellulose pulp produced by this process.
Preferably steps (a) and (c) are further practiced to sense the pH of the material, and steps (b) and (d) are further practiced to add supplementary chemicals to the material, as needed, to obtain substantially optimum conditions in each of the bleaching stages for the particular bleaching chemical utilized. Typically there is at least a third bleaching stage after the second stage, and the method comprises the further steps of: (e) sensing at least one of the brightness, color or lignin content, and the pH, of the material after treatment in the second stage and before treatment in the third stage, and (f) in response to the sensing from step (e), controlling the quantity of bleaching chemical and supplementary chemicals added to the third stage to achieve a desired brightness.
The entire bleaching sequence and chemical charge in the basic method described above can be optionally determined by the sensing of step (a) without further sensing between treatments.
In order to provide a greater variety for treatment, at least one of the bleaching stages has at least a first and second high intensity mixers with chemical addition to each, and there is the further step, in response to sensing in step (a) or step (c), of controlling the flow of material so that it passes only through the first mixer with first chemical addition, or passes through the first and second mixers with chemical addition in each, before the material passes to the next stage.
The first and second bleaching chemicals come from first and second chemical supplies, and the method typically comprises the further steps of sensing the concentration of the chemicals in each of the chemical supplies, and using this information in determining how much chemical to add to the material in the practice of steps (b) and (d). Also there may be the further step of sensing at least one of the brightness, color, or lignin content (e.g. all three) of the material after the last bleaching stage, and using this information to adjust the chemical additions in at least one of the stages for subsequent material being bleached.
Steps (a) and (c) may also be further practiced to sense the L*, a*, or b* indicators of the comminuted cellulosic fibrous material, and steps (b) and (d) may be further practiced to utilize the sensing the L*, a*, or b* indicators to enhance treatment of the material. As described in the publication "The Measurement and Control of Optical Properties of Paper" by Popsen et al (Technidyne Corporation, New Albany, Ind.), L* is an indication of lightness or darkness, a* is an indication of "greenness" or "redness", and b* is an indication of "yellowness" or "blueness". With respect to pulp bleaching, "L*" is analogous to ISO or GE brightness; a high L* value is desirable. At the same time a value of "a*" and "b*" approaching zero is desirable, i.e., as little color as possible. At the present time laboratory scale instrumentation is available to determine these indices (available from Technidyne Corporation), and instrumentation is available for paper machine applications, however on-line indicators for industrial application to pipelines or washers are not believed commercially available. However such on-line equipment is technically feasible and may be utilized in the practice of the invention in place of laboratory scale instrumentation once developed.
The invention also relates to apparatus for bleaching material having cleanliness which vary significantly over time. The apparatus comprises the following elements: A first bleaching stage, and a second bleaching stage. First means for sensing at least one of the brightness, color or lignin content of the comminuted cellulosic fibrous material prior to treatment in the first bleaching stage. Means for controlling the quantity of first bleaching chemical added to the first stage in response to the first sensing means to achieve a target brightness increase for treatment of the material in the first stage. Second means for sensing at least one of the brightness, color or lignin content of the comminuted cellulosic fibrous material after treatment in the first bleaching stage and before treatment in the second stage. And, means for controlling the quantity of second bleaching chemical added to the second stage in response to the second sensing means to achieve a target brightness increase for treatment of the material in the second stage.
The invention also deals with a somewhat related problem that occurs when a stream of pulp to be bleached has different components which consume vastly different amounts of bleaching chemical, differences great enough to result in non-uniform consumption of bleaching chemical. Again this typically occurs in recycled paper utilization facilities where the furnish is often a combination of white paper which typically has a kappa number of less than twenty, often less than ten, and sometimes less than one; colored paper which typically has a higher kappa number than white paper, brown paper which has a much higher kappa number (e.g. over 60), and newsprint, which has a very high kappa number (typically over 100). According to the invention it has been found that when material with such widely varying lignin contents is continuously or intermittently subjected to mechanical action, such as by utilizing a low intensity mixture or a series of low intensity mixers, and/or a retention vessel with one or more agitators, the bleaching results are vastly improved. The method according to this aspect of the invention comprises the following steps: (a) Intensely mixing a first stream of comminuted cellulosic fibrous material having at least two significant (i.e. at least 20% of the total weight) components which consume large differences in bleaching chemical (i.e. at least a kappa number difference of at least four and typically over 10) with bleaching chemical to provide a substantially homogenous mixture of fibrous material and bleaching chemical. And, (b) continuously or intermittently subjecting the mixture to mechanical action of sufficient intensity and duration so as to enhance distribution and redistribution of bleaching chemical in the fibrous material to enhance the uniformity of chemical distribution in the fibrous material until the bleaching action of the bleaching chemical is substantially complete. The invention also relates to an improved pulp produced by this process.
After step (b), steps (a) and (b) can be repeated for a number of other different (or the same) bleaching chemicals, such as the non-chlorine bleaching chemicals described earlier, or chlorine containing chemicals. While the invention is particularly applicable to mixtures of white recycled paper with other types of paper, it also can be used for mixtures of hardwood and softwood virgin fibers which themselves have vastly different lignin contents. Of course the recycled paper is re-pulped, deinked and cleaned before being treated.
The invention also relates to an apparatus for bleaching material having two significant components which consume large differences in bleaching chemical. This apparatus typically comprises means for intensely mixing a first stream of comminuted cellulosic fibrous material having at least two significant components which consume large differences in bleaching chemical with bleaching chemical to provide a substantially homogenous mixture of fibrous material and bleaching chemical; and means for continuously or intermittently subjecting the mixture to mechanical action of sufficient intensity and duration so as to enhance distribution and redistribution of bleaching chemical in the fibrous material to enhance the uniformity of chemical distribution in the fibrous material until the bleaching action of the bleaching chemical has substantially terminated. The means for intensely mixing typically comprises a high shear mixer such as a Kamyr, Inc. MC.RTM. mixer, or an Ahlmix.TM. mixer. The low intensity mixing means preferably comprises a plurality of in-series low intensity mixers, either a static or moving parts type, and/or a retention vessel with at least one agitator.
It is the primary object of the present invention to provide for the improved bleaching of comminuted cellulosic fibrous material having cleanliness which vary significantly over time and/or having at least two significant components which consume large differences in bleaching chemical. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.